1. Field of the Inventions
The disclosed invention relates generally to integrated circuit fabrication, techniques for fabrication of computer memory, and masking techniques.
2. Description of the Related Art
As a consequence of many factors, including demand for increased portability, computing power, memory capacity and energy efficiency in modern electronics, integrated circuits are continuously being reduced in size. To facilitate this size reduction, research continues into ways of reducing the sizes of integrated circuits' constituent features. Examples of those constituent features include capacitors, electrical contacts, interconnecting lines, and other electrical devices. The trend of decreasing feature size is evident, for example, in memory circuits or devices such as dynamic random access memories (DRAMs), static random access memories (SRAMs), ferroelectric (FE) memories, electronically-erasable programmable read-only memories (EEPROMs), flash memories, etc.
Computer memory typically comprises millions of identical circuit elements, known as memory cells, arranged in a plurality of arrays with associated logic circuitry. Each memory cell traditionally stores one bit of information, although multi-level cell devices can store more than one bit per cell. A conventional DRAM memory cell typically consists of two electrical devices: a storage capacitor and an access field effect transistor. Each memory cell is an addressable location that can store one bit (binary digit) of data. A bit can be written to a cell through the transistor and read by sensing charge on the storage electrode from the reference electrode side. One common type of computer memory that can benefit from higher density components is DRAM. By decreasing the sizes of constituent electrical devices, the conducting lines that connect them, and the conductive contacts carrying charge between them, the sizes of the memory devices incorporating these features can be decreased. Note that some memory arrangements can combine the storage and switch functions in a single device (e.g., dendritic memories using silver-doped chalcogenide glass elements) and others can omit the access device (e.g., magnetic RAM). Storage capacities and circuit speed can be increased by fitting more memory cells into the memory devices.
The concept of pitch can be used to describe the size of integrated circuit features. Pitch is the distance between identical points in two neighboring features. These features are typically defined by spaces between adjacent features, which spaces may be filled by a material, such as an insulator. As a result, pitch can be viewed as the sum of the width of a feature and of the width of the space separating that feature from a neighboring feature. When the pitch is reduced, features are more dense—that is, more features can fit onto the same surface. Although it may seem counterintuitive, the term “pitch reduction” is synonymous with “pitch multiplication” in common usage. That is, conventionally “multiplication” of pitch by a certain factor actually involves reducing the pitch by that factor. The term “pitch multiplication” emphasizes that more features can fit in the same area, while the term “pitch reduction” emphasizes that in order for more features to fit, those features are generally smaller. Pitch thus has at least two meanings: the linear spacing between identical features in a repeating pattern; and the density or number of features per linear distance. The conventional terminology is retained herein.
The demand for continual reduction in feature sizes places ever greater demands on techniques used to form the features. For example, photolithography is commonly used to pattern features on a substrate. Certain photoresist materials only respond to certain wavelengths of light. One common range of wavelengths that can be used lies in the ultraviolet (UV) range. Because many photoresist materials respond selectively to particular wavelengths, photolithography techniques each have a minimum pitch below which that particular photolithographic technique cannot reliably form features. This minimum pitch is often determined by the wavelength of light that can be used with that techniques. Thus, the minimum pitch of a photolithographic technique can limit feature size reduction.
Pitch multiplication can extend the capabilities of photolithographic techniques to allow creation of more densely arranged features. Such a method is described in U.S. Pat. No. 5,328,810, issued to Lowrey et al., the entire disclosure of which is incorporated herein by reference and made part of this specification.
The critical dimension (CD) of a mask scheme or circuit design is the scheme's minimum feature dimension, or the measurement of the smallest width of the smallest feature that exists in that design or scheme. Due to factors such as geometric complexity and different requirements for critical dimensions in different parts of an integrated circuit, typically not all features of the integrated circuit will be pitch multiplied. Furthermore, pitch multiplication entails many additional steps relative to conventional lithography; the additional steps can involve considerable additional expense. Pitch multiplication often provides less control over the resulting features than that provided by direct patterning without pitch multiplication. Thus, pitch multiplication is typically thought useful for regularly spaced lines, such as conductive lines for a memory array. On the other hand, typical micromasking techniques, such as isotropic shrink steps, can result in a reduction in feature size but no corresponding increase in feature density. There have also been challenges in transferring very fine patterns to underlying layers because existing techniques do not adequately maintain resolution and fidelity through the transfer. There is a need for methods that can allow for smaller and more efficient operative units on an integrated circuit; such methods will advantageously increase feature density and decrease chip size.
Thus, there is a need for a reduction in the size of integrated circuits and an increased operable density of the arrays of electrical devices on computer chips.